Export 16-bit image type aliases

[RunDevelopment]

Resolves #1384

This PR makes the type aliases for 16-bit image types publically accessible.

Why?

• Consistency. The aliases of the types of other DynamicImage variants are already exposed.
• Easy of use. Users could define their own types, but not having access to them is just a little stumbling block users have to navigate. (Something I ran into myself, when I first used this library.)